1. Field of the Invention
The present invention relates to a six-stroke engine in which after an exhaust stroke, a piston makes one reciprocating motion, and an intake stroke is then executed, and a method of operating the six-stroke engine.
2. Description of the Related Art
An engine is required to improve fuel consumption and output. To implement this, the thermal efficiency of the engine needs to be further improved.
The thermal efficiency of the engine is able to be improved by facilitating cooling of the cylinder, advancing the ignition timing, or raising the compression ratio. An example of a conventional engine that cools the cylinder is a six-stroke engine in which after an exhaust stroke, the piston makes one reciprocating motion, and an intake stroke is then executed.
The six-stroke engine sequentially executes six strokes, that is, four strokes including an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke and additional scavenging strokes including a second intake stroke and a second exhaust stroke to facilitate cooling.
The six-stroke engine of this type mainly has two problems, as will be described below. As the first problem, since the intake valve opens in the second intake stroke, and new air is inhaled, a pumping loss occurs. As the second problem, the exhaust valve opens in the second exhaust stroke, and air that contains oxygen unconsumed by combustion is discharged to the exhaust passage. The oxygen amount in the air is detected by an O2 sensor in the exhaust passage, and the air-fuel ratio is calculated as an abnormal value. In addition, since an excessive amount of oxygen flows into the catalyst in the exhaust passage, the exhaust gas cannot sufficiently be cleaned.
These problems can be solved by using a six-stroke engine operating method described in Japanese Patent Laid-Open No. 2007-303303. The six-stroke engine operating method disclosed in Japanese Patent Laid-Open No. 2007-303303 is executed by keeping the exhaust valve open from the “exhaust stroke” up to the start of the “intake stroke”, instead of executing the second intake stroke and the second exhaust stroke. That is, the exhaust valve is not closed even after the exhaust stroke and is kept open from the end of the cooling period to the start of the intake stroke.
According to this method, when the piston lowers after the exhaust stroke, the burned gas in the exhaust passage is inhaled into the cylinder. The burned gas in the cylinder is discharged to the exhaust passage in the subsequent piston rise process.
When this operating method is used, it is possible to cool the interior of the cylinder using the burned gas that is temporarily discharged to the exhaust passage to lower the temperature while suppressing a pumping loss. Japanese Patent Laid-Open No. 2007-303303 also discloses an operating method for further cooling the interior of the cylinder by injecting water into the cylinder during the cooling period between the exhaust stroke and the intake stroke.
The six-stroke engine operating method disclosed in Japanese Patent Laid-Open No. 2007-303303 mainly has the following three problems.
As the first problem, the interior of the cylinder is not cooled as expected. This is because the difference between the temperature in the cylinder and the temperature of the burned gas inhaled into the cylinder during the cooling period is small.
As the second problem, a large quantity of burned gas flows into the cylinder during the cooling period. That is, since the intake stroke starts in a state in which a large amount of burned gas remains in the combustion chamber, the ratio of the burned gas in the cylinder is high after the end of the intake stroke in many operation ranges. Additionally, at the time of valve overlap when starting the intake stroke, it is highly possible that the burned gas in the combustion chamber flows back to the intake passage due to a pressure difference.
The third problem arises in a case in which the operating method of injecting water into the cylinder to enhance the cooling effect is used. That is, the engine needs to be equipped with auxiliary units such as an injector for water injection and a water storage tank, resulting in a bulky engine.